Pasteurisation and sterilisation of liquid food products such as milk or fruit juices is usually done by convective heating processes, such as a batch-wise retort cooking or continuous HTST (High Temperature Short Time) processes.
Heating of food products by means of an electric field treatment has been proposed.
In U.S. Pat. No. 6,214,297 for example is disclosed an pulsed electric field (PEF) treatment of food products, wherein a food product in a treatment chamber that forms part of an electric circuit, is subjected to high-voltage bipolar pulses. The aim of the treatment is to induce stress and mortality in biological cells. The electric circuit used in the process of U.S. Pat. No. 6,214,297 comprises an energy storage component, typically a capacitor, for storing electrical energy that is discharged from time to time as an electrical pulse to the food product.
In WO2011/139144 is disclosed a method and system for treating a solid food product, wherein the food product is subjected to a pulsed electric field for the disruption of the cells of the food product (so-called e-cooking). As in the process of U.S. Pat. No. 6,214,297, a pulsed electrical field is used. In the process of WO2011/139144, a relatively low field strength is used. Between the pulses, there is a pause time, typically of 0.1 seconds.
In H. L. M. Lelieveld, Encyclopedia of Biotechnology in Agriculture and Food (2011) p. 575-578, is mentioned pulsed electric field processing of orange juice to achieve the same effect as in thermal pasteurisation.
In WO2009/111640 is disclosed a system and method for electrically heating a liquid, wherein the liquid is subjected to alternating current or to pulsed direct current at a high frequency. In the method of WO2009/111640 an electrical current is generated through the liquid to be treated that is sufficient to produce resistive heating of the liquid and to break at least some molecular bonds of molecules defining the liquid. The method of WO2009/111640 is used for heating and at the same time treating water of e.g. hot tubes and swimming pools. Both resistive heating and electrolysis occurs. The water is thus simultaneously heated and treated by the free oxygen obtained in electrolysis of the water or by free chlorine obtained in the electrolysis of salt added to the water.
In U.S. Pat. No. 5,863,580 is disclosed a method for resistive heating of liquid egg, wherein the product is pasteurised without substantial electrolysis. In the method of U.S. Pat. No. 5,863,580, the material to be treated has a high electrical resistance, i.e. at least 100, but preferably more than 1000 ohms, and the method is thus operated at low current levels. Also, electrical current of a low frequency is used, i.e. 400 Hz or lower, preferably below 100 Hz.
A disadvantage of using low frequencies is that this will lead to electrode degeneration and thus to undesired poisoning of the liquid product to be heated. In the method of U.S. Pat. No. 5,863,580, special measures are taken to lower the current density in the fluid close to the electrodes.
Although the concept of electrical heating of liquid products has been mentioned in the prior art, a process for electrical heating of liquid products has not yet been reduced to practice.